The stators and rotors of variable reluctance machines have magnetic saliencies commonly known as salient poles. Such a configuration is commonly called "doubly salient" as illustrated in FIG. 1. Each stator pole 2 is surrounded by a winding of one or more turns of electrically conductive material and appropriate insulation. A phase winding 3 is a pair of series connected windings respectively wound on diametrically opposed poles 2. Only one phase winding 3 is illustrated with it being understood that the remaining pairs of poles each have a phase winding wound on them. The phase windings 3 are grouped together so that a balanced torque is produced in the machine when the windings are excited from an external source of electrical energy and also so that voltage and current requirements of the external source are satisfied. There are no windings of any type or magnets associated with the machine rotor 4. The number of poles 2 on the stator 5 differ from the number of poles 6 on the rotor 4. When the rotor 4 is rotated with respect to the stationary stator poles 2, a variation in reluctance is observed in stator poles. This variation in reluctance is observed as a variation in the inductance of the phase windings 3 which can be readily measured by appropriate instrumentation. Starting from the condition of a stator pole 2 being exactly half way between two rotor poles 6, known as the "unaligned position", the inductance of the phase winding 3 has its minimum value. The unaligned condition in most variable (switched) reluctance machines generally exists throughout an arc of several degrees of rotor rotation. The inductance of the phase winding 3 is fairly constant at its minimum value throughout this arc. Excitation of the phase windings 3 during this rotational period of constant, minimum inductance results in negligible developed torque. As the rotor 4 turns beyond the arc of minimum inductance, the inductance measured in the phase winding 3 increases to a maximum value which is when a pair of rotor and stator poles 2 and 6 are exactly aligned, known as the "aligned position" as illustrated in FIG. 1. When the stator winding 3 is excited with an electrical current as the inductance is increasing from minimum to maximum motor torque is developed on the machine shaft 7. When the phase winding 3 is excited as the inductance is decreasing from maximum to minimum, torque of the opposite direction is developed on the shaft 7. This torque is often termed "generator torque" or "regenerative torque", the latter term being associated with a motor in a braking mode.
In modern variable reluctance machines, switching of the phase windings 3 is accomplished by solid state switching devices generally known as power semiconductors. Specific switching devices include thyristors, transistors, MOSFETS, IGBT's, and many other devices including combinations of the above-mentioned devices. In general, the power semiconductors are operated in an "on/off" mode rather than a linear mode associated with linear amplifiers. The switching times of the power semiconductors are determined by a "logic system" composed of small signal electronic devices and circuits similar to modern computer circuits and systems. The logic system operates in response to various sensors which sense such machine parameters as the position of the rotor poles with respect to the stator poles, current levels in the windings, voltage levels, or other signals required for the desired operation and protection of the machine.
FIG. 2 illustrates a first prior art machine control circuit 10 for driving a four phase variable reluctance machine. Phase windings 12 are sequentially connected between a positive potential 16 and a negative potential 18 of DC power source 14 by closing of switches 20 and 22 which are controlled by a logic (switch) controller (not illustrated). At any one time only one phase winding 12 is energized by its associated switches 20 and 22 being in the on state under the control of control signals applied by the aforementioned logic controller. Freewheel diode 24 is forward biased when the logic controller applies control signals to the switches 20 and 22 to stop the flow of current in the phase winding 12 to which the switches are connected. An induced potential caused by the turning off of the switches 20 and 22 forward biases the freewheel diode 24 and the first diode 26 by inducing a positive potential at terminal 28 of the phase winding 12. The positive potential at terminal 28 causes current flow through the first freewheel diode 24, through the power supply 14 and through diode 26 back to terminal 30 of the phase winding 12. The current flowing in each of the phase windings 12 occurs sequentially under the control of the aforementioned lo logic controller as described above. The motor drive circuit 10 of FIG. 2 has a disadvantage of requiring a pair of switches 20 and 22, a freewheel diode 24 and a first diode 26 for each phase winding 12.
FIG. 3 illustrates a second machine control circuit 40 for a variable reluctance machine. Like reference numerals identify like parts in FIGS. 2 and 3. The number of transistor switches 42 and 44 is equal to the number of phase windings 12. A second diode 44 is connected in series with each of the phase windings 12 to control the flow of current between a pair of switches 42 and 44 which are coupled to different sides of a phase winding 12. A logic (switch) controller (not illustrated) sequentially turns on the pairs of switches 42 and 44 connected to opposite sides of the phase windings to control the flow of current in the phase windings to cause the rotor to rotate. The circuit of FIG. 3 has the disadvantage that the diodes 44 in series with each of the phase windings 12 cause a power loss. Furthermore, the circuit 40 of FIG. 3 does not operate well in an overlapping mode of operation.